I have a vent/fan in the masrer bath that travels 3 feet and exits through an outside wall.
On the exterior there is a louver flap vent.....3 louvers that are blown open when the fan is on, then fall closed by gravity when the fan is off.
I assume that there is also a draft preventer of some kind in the fan itself.
On cold or windy days, the bathroom will be as much as 10 degrees cooler than the bedroom and I can feel air coming in through the vent.
Is there anything I can do to seal the vent when the fan is not on???

It is a 3 year old house so the fan unit is fairly new.